Modelling of Pollutant Transport in the Atmosphere - MANHAZ

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dispersion. The buoyancy of the effluent itself can be big importance, if it’s a cold gas it will fall to the ground (see part II: Atmospheric Dispersion: Heavy gases) or it can be hot or energetic, in which case we need to consider plume and momentum rise. Inversion height. The inversion height and its strength is another very important parameter to consider for atmosphere dispersion. Source types may alter between single point sources and line sources over explosive sources with huge heat and momentum content. The Chernobyl accident was in its initial phase characterized by a heat and vapor explosion, followed by a plume release with significant heat content. On the Ukrainian night of April 26 1986, the burning graphite plume rose some ~1 km aloft before it blew north-westwards in a strongly stable stratified atmosphere. It is first today are we about to understand and model the combined effects of the Chernobyl initial stage release and the stable stratified atmosphere in connection with local inversion heights and local wind sheer. Effluents can also be of continuous type continuous (plumes) or instantaneous (puffs). In addition, this affects the resulting dispersion pattern. Atmospheric Dispersion at different scales Surface layer scaling The planetary Surface Layer (SL) was earlier defined as the lowest ~100 meters of the atmosphere. It is characterized by its approximately constant vertical fluxes of heat and momentum. The magnitudes of these fluxes can therefore be determined by measurements near the surface (typically at 10 meters height). It is only within the PBL, which is within the thin surface layer of the atmosphere that the surface layer Monin-Obukhov similarity scaling of dispersion is applicable. Mesoscale Meso is Greek for " in between" and refers to atmospheric phenomenon which occurs on horizontal scales large enough that the hydrostatic approximation (i.e., that the vertical acceleration forces of the air mass can be neglected compared with the hydrostatic pressure forces) is a valid assumption (usually it is on scales bigger > 2 km); yet small enough (
dient winds can approximate the regional wind circulation [Pielke (1984)]. Vertically, the surface layer extends a few hundred meters whereas the mesoscale extends throughout the entire troposphere. Mesoscale models utilize grid spacing ranging between a few meters for modelling of local scale dispersion phenomena and out to 20 km or more in numerical weather forecast models utilized on the regional scale. The corresponding time scales ranges from fractions of a second in the nearsurface layer concentration fluctuations to tens of hours for the meso-γ scale weather system calculations. Atmospheric dispersion of a puff progresses with different scaling laws on the various scales. In the start, within the SL, is expands at first under influence of the initial source dimension, the heat and momentum content of the release, by nearby building effects, by down-wind changes in roughness and surface heatfluxes (in particular by the atmospheric stability). Then it undergoes diffusion by transition through various time and spatially changing boundary layers. Additional local scale effects include heterogeneous surfaces, hills, and rolling and steep complex terrain. When a dispersing puff reaches the Mesoscale ( say when the puff size is comparable to the mixing height (~2 km) , the expansion will furthermore be influenced by winds originating from differential heating such as: sea-and land breezes, up and down slope winds, valley circulation's and orographically induced winds. Long Range After a day of travel time or so, most plumes and puffs have diffused to a lateral extent of the order of ~100 km, that is, they have diffused horizontally to scales where the atmosphere for all practical purposes have become two- dimensional. Turbulence on this scale also takes on a different form known as largescale enstrophy cascade. The usual local scale plume or puff confined entity picture soon disappear for two-dimensional turbulence, and the usual surface and boundary layer parametric descriptions of dispersion is no longer valid. 11
Page 1 and 2: MANHAZ position paper on: Modelling
Page 3 and 4: Troposphere. Trapped between the st
Page 5 and 6: The importance of the local wind fi
Page 7 and 8: Figure 2. Typical vertical wind spe
Page 9: Notice that this source of turbulen
Page 13 and 14: y Russian and British researchers t
Page 15 and 16: σ () t = 2K t + σ () 0 2 2 z z z
Page 17 and 18: Figure 3 Pasquill stability classif
Page 19 and 20: Monin-Obukhov similarity scaling (z
Page 21 and 22: With Richardson's diffusivity KDN i
Page 23 and 24: The corresponding predicted puff gr
Page 25 and 26: The cyclic variation of the stabili
Page 27 and 28: Dispersion modelling has also high
Page 29 and 30: structure of the diffusion. In prin
Page 31 and 32: Atmospheric dispersion module for n
Page 33 and 34: LSP is a local scale pre-processing
Page 35 and 36: It consists of nested modules, whic
Page 37 and 38: land uses the nearest land-based Me
Page 39 and 40: Another example, showing the effect
Page 41 and 42: Density calculations. Numerical dis
Page 43 and 44: cloud is expected to cover a larger
Page 45 and 46: Auxiliary parameters like concentra
Page 47 and 48: Variable atmospheric wind condition
Page 49 and 50: Busch, N.E., On the Mechanics of At
Page 51 and 52: Monin A.S. (1959) Smoke propagation
Page 53 and 54: Goldwire, H. C., McRae, T. G., John
Page 55: Table of Content Modelling of Pollu

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